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Description/Abstract

The onset of continent-wide glaciation in Antarctica is still poorly understood, despitebeing one of the most important palaeoclimatic events in the Cenozoic. The Eocene/Oligocene boundary interval has recently been recognized as a critical time for Antarcticclimatic evolution, and it may mark the preglacial–glacial transition. Magnetic susceptibility,intensity of natural and artificial remanences, hysteresis parameters and magneticanisotropy of the lower half ( late Eocene/early Oligocene) of the CIROS-1 core (fromMcMurdo Sound, Antarctica) reveal alternating intervals of high and low magneticmineral concentrations that do not correspond to lithostratigraphic units in the core.Pseudo-single-domain magnetite is the main magnetic mineral throughout the sequence,and sharp changes in magnetite concentration match changes in clay mineralogybeneath and at the Eocene/Oligocene boundary. The detrital magnetite originated fromweathering of the Ferrar Group (which comprises basic extrusive and intrusive igneousrocks). Weathering processes and input of magnetite to the Victoria Land Basin wereintense during periods when the Antarctic climate was warmer than today, but duringintervals when the climate was relatively cool, chemical weathering of the Ferrar Groupwas suppressed and input of detrital magnetite to the Victoria Land Basin decreased.Our results also indicate that a cold and dry climate was not established in Antarcticauntil the Eocene/Oligocene boundary, with major ice sheet growth occurring at theearly/late Oligocene boundary. Some earlier cold intervals are identified, which indicatethat climate had begun to deteriorate by the middle/late Eocene boundary.